Road antenna controlled on the basis of receiving rate

ABSTRACT

A road antenna apparatus includes a road antenna  104  which is mounted on a post  103  and at an elevated position on a road R and establishes radio communication with an on-vehicle radio device  102  mounted in a vehicle  101  which is traveling over the road; and a laser-beam emitting device  111  which is mounted on the road antenna and radiates a laser beam onto a predetermined position  113  on the surface of the road. An offset in the angle at which a road antenna is mounted can be readily ascertained on the basis of a distance between a predetermined position on the surface of the road and a position  114  on the road surface onto which a laser beam is actually radiated.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a road antenna for use with anelectric toll collection (ETC) system, which system can automaticallycollect a toll through radio communication without involvement oftemporary stopping of a traveling vehicle which is passing through atollgate of a turnpike.

[0002] The present invention also relates to a transmitter, a receiver,a radio system, and a method of setting a communications area, all ofwhich are applied to narrow-band communication, such as that realized bya turnpike electric toll collection system (hereinafter referred tosimply as an “ETC system”), and which controls an output of radiotransmission established between a cell station and a mobile station.

[0003] Further, the present invention relates to a travel-speed supportsystem which determines whether or not a vehicle is traveling in excessof a speed limit for vehicles set on a turnpike or an ordinary road andsends a notice to the driver of the vehicle when the vehicle istraveling in excess of the speed limit, as well as to an antenna for usewith the system.

[0004] A traveling vehicle has conventionally been required totemporarily stop at a tollgate of a turnpike and receive a highwayticket from or pay a toll to an official, thus greatly contributing to atraffic jam. Against such a backdrop, attempts have been made to put anelectronic toll collection system (ETC) into actual use as a nonstoptollgate system which eliminates a necessity for temporarily stopping avehicle.

[0005]FIG. 27 shows an example ETC system scheduled to be put intopractical use. In this drawing, a vehicle 1 is equipped with anon-vehicle radio device 2. A road antenna 4 is mounted on a post 3 andat a position above a road R. Radio communication is established betweenthe on-vehicle radio device 2 and the road antenna 4. A vehicle sensor 5is disposed on either side of the road R for optically detecting passageof the vehicle 1.

[0006] The antenna 4 establishes radio communication with on-vehicleradio device 2 mounted in a vehicle 1 which is passing through the post3, to thereby specify the owner of the vehicle 1 through use of theradio device 2. For example, ID information to be used for specifyingthe owner of the vehicle 1 is written in the on-vehicle radio device 2.

[0007] A toll and information for specifying the owner of the vehicle 1are written into a storage area of the antenna 4 every time the vehicle1 passes through the post 3. The toll and the vehicle owner IDinformation, which have been acquired while the vehicle 1 passes throughthe post 3, are transmitted to an unillustrated center by way of theantenna 4. The unillustrated center summarizes tolls and on a monthlybasis collects the tolls from the owner of the vehicle 1 that has passedthrough the post 3.

[0008] In this system, after a vehicle detector 5 disposed on the roadof a turnpike has detected passage of the vehicle 1, radio communicationpertaining to a toll is established between the antenna 4 and theon-vehicle radio device 2. Accordingly, collection of tolls is performedsmoothly without involvement of temporary stopping of a travelingvehicle.

[0009] In terms of design of the ETC system, there is specified acoverage area of radio communication established between the on-vehicleradio device 2 and the road antenna 4. FIG. 28 is a plan view showing anexample coverage area. A hatched communications area F1 is a rangewithin which radio communication can be established between theon-vehicle radio device 2 and the road antenna 4. The remaining area;i.e., a non-response area F2, is a range in which radio communication isnot permitted.

[0010] An electric field level of the road antenna 4 chiefly determineswhether or not radio communication is feasible. In a case where theelectric field of the road antenna 4 is greater than a predeterminedlevel, the on-vehicle radio device 2 can perform a receiving operation,thus enabling radio communication. In contrast, in a case where theelectric field of the road antenna 4 is less than a predeterminednon-response level, the on-vehicle radio device 2 cannot perform areceiving operation. Accordingly, the area where the on-vehicle radiodevice 2 cannot establish radio communication is taken as a non-responsearea.

[0011] In the previously-described case, the road antenna 4 has a sharpdirectional pattern, and an angle at which the road antenna 4 is mountedon the post 3 greatly affects the distribution of electric field. FIG.29 shows an example road antenna 4 mounted on the post 3. FIG. 30 showsan example distribution of-receiving electric field at a position 1meter elevated from the road R and with respect to the direction inwhich the vehicle travels.

[0012] As shown in FIG. 30, an electric field level L1 designates acommunicable threshold level, and an electric field level L2 designatesa non-response threshold level. From FIG. 30, it is understood that thecommunications area F1 and the non-response area F2, which are shown inFIG. 28, are embodied by reference to these threshold levels.

[0013]FIG. 31 shows an example distribution of an electric fieldproduced in a case where only an angle θ at which the road antenna 4 ismounted and is shown in FIG. 29 is changed. In this case, thepredetermined communications area F1 shown in FIG. 28 is not ensured,and receiving power—which is greater than the communicable thresholdvalue level L1 and at which the on-vehicle radio device 2 can perform areceiving operation—exists in the non-response area F2. There is apossibility of the ETC system yielding a failure.

[0014] For example, as shown in FIG. 32, in a case where a vehicle 1Ahaving no on-vehicle radio device and a vehicle 1B having an on-vehicleradio device passe through the ETC system while the vehicle 1B isfollowing close behind the vehicle 1A, the vehicle sensors 5 detect thevehicle 1A. However, radio communication is established between the roadantenna 4 and the on-vehicle radio device 2 of the vehicle 1B. As aresult, the ETC system yields a failure, thereby permitting passage ofthe vehicle 1A without charge.

[0015] In order to prevent a failure, means for ascertaining in advancean angle θ at which the road antenna 4 is mounted (hereinafter referredto simply as a “mount angle”) becomes necessary. At the time ofinstallation of the road antenna 4, the post 3 standing at a height of 5m or more is fixed through use of a bucket vehicle or a like vehicle.After installation of the road antenna 4, the mount angle θ of the roadantenna 4 cannot be readily ascertained. However, it is thought thatafter installation the mount angle θ of the road antenna 4 may bechanged by a blow or an earthquake.

[0016]FIG. 33 is a plan view showing an example coverage area. As shownin FIG. 33, in terms of design of the ETC system, there is specified acoverage area of radio communication established between the on-vehicleradio device 2 and the road antenna 4. A communications area F1 is arange within which radio communication can be established between theon-vehicle radio device 2 and the road antenna 4. The remaining area isa range in which radio communication is not permitted. A

[0017] In the previous ETC system, the communications area F1 must becovered by means of the directivity of the road antenna 4. However, thetransmission power of the road antenna 4 is changed for reasons ofenvironmental or secular changes, the range of the communications areaF1 is also changed, thereby resulting in a system failure. Further,depending on variation in the angle at which the road antenna 4 ismounted, the communications area F1 is greatly changed, therebyinterfering with radio communication established by a vehicle which istraveling on an adjacent lane.

[0018]FIG. 34 shows a commonly-employed transmission circuit 50. In FIG.34, reference numeral 51 designates a radio section; 52 designates alevel control attenuator; and 53 designates an antenna.

[0019] The transmission circuit 50 is applied to, for example, an ETCsystem. According to this system, a narrow-band communications area isformed in the space between radio devices disposed on either road of aturnpike. Radio communication is established between a traveling vehicleand the road radio devices through use of a radio wave of predeterminedfrequency (for example, a frequency band of 5.8 GHz), to thereby collecta toll for using the turnpike.

[0020]FIG. 35 shows an antenna disposed at a tollgate of an ETC system.In FIG. 35, reference numeral 61 designates a road antenna; 62designates an island; 63 designates a lane; and 64 designates acommunications area. For example, a vehicle which is traveling in, forexample, a lane 63 a, establishes communication with a road antenna 61 awithin only a communications area 64 a.

[0021] In terms of prevention of a chance of interference arising in anradio wave used in an adjacent lane, or prevention of erroneouscommunication with another vehicle running before or after the vehicleof interest in the same lane, the range of communications area 64preferably remains constant. For this reason, a transmission e.i.r.pvalue output from the antenna 53 shown in FIG. 34 must be set to apredetermined level.

[0022] However, variations are present in constituent elements of thetransmission circuit 50; that is, the transmission output of the radiosection 51 or the antenna gain-of the antenna 53. In order to obviatethese variations, individual constituent elements: must be adjustedthrough use of the level control attenuator 52.

[0023] The road antenna has a directional pattern such as that shown inFIG. 36, and a communications area of the road antenna differs accordingto an angle at which the antenna is mounted. Consequently, the anglemust be adjusted in order to ensure a desired communications area.Measurement of receiving field intensity at each angle requires a greatdeal of manpower.

[0024] Moreover, the ETC system must ensure highly-reliablecommunication. To this end, a communication area in which radiocommunication is to be established and a non-response area in which noradio communication is to be established must be embodied in compliancewith specifications of system design. Therefore, such specifications areusually accomplished by imparting a sharp directional pattern to theroad antenna.

[0025] However, the radio wave emitted from the road antenna or theon-vehicle device spreads not only to a lane of interest but also to theopposite lane, because of multiple reflections of a radio wave inducedby vehicles or surrounding facilities. Therefore, radio communication iserroneously established with an oncoming vehicle to which a charge isnot allowed to be charged, and a toll may be erroneously charged to anoncoming vehicle.

[0026] Further, the ETC system eliminates a necessity of temporarilystopping a vehicle at a tollgate. However, a traveling vehicle may passthrough a tollgate at high speed or keep traveling at the same speedeven after the vehicle has entered an ordinary road. Thus, a vehiclebecomes apt to induce a traffic accident. In order to prevent a trafficaccident, there is needed a travel-speed support system for measuring atravel speed of a vehicle which is traveling on a road adopting an ETCsystem, to thereby realize smooth travel.

[0027] In association with actual use of a turnpike ETC system, anecessity for temporarily stopping a vehicle at a tollgate iseliminated. As a result, it is predicted that a traveling vehicle passesthrough a tollgate at high speed or enters an ordinary road from aturnpike without being aware of a change in legal speed.

[0028] Moreover, in order to avoid establishment of radio communicationwith a vehicle which is traveling in an adjacent lane, the ETC systemestablishes radio communication at a frequency of 5.8 GHz within anarrow communications area F1 formed by the road antenna 4.

[0029]FIGS. 37A and 37B show the directional patterns of the roadantenna. FIG. 37A shows a horizontal directional pattern of the roadantenna 4, and FIG. 37B shows a vertical directional pattern of the roadantenna 4. As is evident from these characteristic plots, the roadantenna 4 shows horizontal and vertical directional patterns in which acommunication area can be formed within a narrow range of −20 to +20degrees relative to the center.

SUMMARY OF THE INVENTION

[0030] The present invention has been conceived to solve such a drawbackof the background art-and is aimed at providing a road antenna in whichan angle at which the road antenna is mounted can be readily ascertainedafter the road antenna has been mounted on a post.

[0031] The present invention has been conceived to solve such a drawbackof the background art and is aimed at providing a road antenna which canprevent occurrence of a change in a communications area by means ofcontrolling the road antenna and prevent occurrence of a system failureor interference of radio communication established by a vehicletraveling on an adjacent lane.

[0032] The present invention has been conceived to solve such a drawbackof the background art and is aimed at providing a transmitter, areceiver, a radio system, and a communications area setting method, allof which enable savings in labor required for measuring field intensityand ensure a desired communications area.

[0033] The present invention has been conceived to solve such a drawbackof the background art and is aimed at providing a road antenna whichprevents occurrence of erroneous communication with an oncoming vehicletraveling in the opposite lane.

[0034] The present invention is aimed at providing a travel-speedsupport system which sends to a vehicle which travels in excess of aspeed limit a warning to reduce travel speed, to thereby preventtraveling of a vehicle at extralegal speeds and support smooth travel ofa vehicle on a turnpike or an ordinary road.

[0035] The present invention has been conceived to solve the drawback ofthe background art and is aimed at providing a road antenna which canform a narrow communications area even when a structure is located at anelevated position above the road antenna.

[0036] According to first aspect of the invention, a road antennacomprises a road antenna which is mounted on a post and at an elevatedposition on a road and establishes radio communication with anon-vehicle radio device mounted in a vehicle which is traveling over theroad; and a laser-beam emitting device which is mounted on-the roadantenna and radiates a laser beam onto a predetermined position on thesurface of the road. An offset in the angle at which a road antenna ismounted can be readily ascertained on the basis of a distance between apredetermined position on the surface of the road and a position on theroad surface onto which a laser beam is actually radiated.

[0037] Preferably, the road antenna according to the first aspectfurther comprises a laser-beam receiving device which is mounted on thepredetermined location on the surface of the road and receives a laserbeam emitted from the laser-beam emitting device, wherein the operationof the road antenna is stopped when the laser-beam receiving devicecannot receive the laser beam. In a case where the laser-beam receivingdevice fails to receive a laser beam emitted from a laser-beam emittingdevice that has been disposed at a predetermined elevated position abovethe road at the time of installation of the road antenna, it becomesevident that a change has arisen in the angle at which the road antennais mounted. Therefore, the operation of the road antenna is stopped inorder to avoid an operation failure of an electric toll collectionsystem.

[0038] According to a second aspect of the invention, a road antennacomprises:

[0039] a road antenna which is disposed at an elevated position above aroad and establishes radio communication with an on-vehicle devicemounted in a vehicle traveling on the road; a receiver which is disposedat a predetermined location on the surface of the road and within acommunications area, receives a radio wave output from the road antenna,and outputs a signal proportional to the power of the radio wave; and acontroller for determining transmission power of the A road antenna onthe basis of the signal output from the receiver, wherein the controllercontrols the road antenna so as to prevent the transmission power of theroad antenna from exceeding a predetermined value. The receiver detectsthe transmission power of the road antenna, and a signal proportional tothe thus-detected transmission power is fed back to the controller, tothereby adjust the transmission power of the road antenna so as toprevent occurrence of a change in the communications area.

[0040] Preferably, receivers are disposed at respective corners of thecommunications area formed on the road, and the controller determines,from signals output from the respective receivers, the angle at whichthe road antenna is mounted, to thereby detect an offset in the angle ofthe antenna with respect to a predetermined angle. The signals outputfrom the respective receivers are fed back to the controller, and thecontroller detects, on the basis of these signals, the angle at whichthe road antenna is mounted, to thereby detect an offset from a presetinitial angle of the road antenna.

[0041] According to third aspect of the invention, the present inventionprovides a method of setting a communications area, comprising the stepsof: measuring a receiving rate for each of frames of a received signalwhen a receiver receives a radio wave transmitted from a transmitter;detecting change in receiving rate on a per-frame basis, the changebeing induced by a change in a transmission output of the radio wavetransmitted from the transmitter; and setting, into the transmitter, atransmission output obtained when there is detected a receiving ratesuitable for a desired communications area established between thetransmitter and the receiver. The method ensures a desiredcommunications area through simple procedures while avoiding manpowerrequired for measuring field intensity.

[0042] According to the fourth aspect of the present invention, a radiosystem comprises: a transmission section including a modulation sectionfor producing a modulation signal, gain controller for controlling atransmission output, a power amplification section for amplifying atransmission signal to a desired level, and an antenna; and a receivingsection including an antenna, frequency converter for converting into anintermediate frequency a high-frequency signal received by way of theantenna, a demodulation section for demodulating the intermediatefrequency, decoder for converging a demodulated signal into digitaldata, and receiving rate detector for detecting a receiving rate foreach of frames of a received signal. On the basis of the receiving ratedetected on a per-frame basis by the receiving rate detector of thereceiving section, the gain controller of the transmission sectionvaries a transmission output. As a result, a desired communications areacan be set in a space between the transmission section and the receivingsection. At this time, measurement of field intensity is not necessary.

[0043] The present invention according to the fifth aspect of theinvention provides a transmitter comprises: a modulation section forproducing a modulation signal; gain controller for controlling atransmission output; a power amplification section for amplifying atransmission signal to a desired level; and an antenna, wherein the gaincontroller varies the transmission output on the basis of a receivingrate for each frame determined when a receiver receives a transmissionsignal. On the basis of the receiving rate detected on a per-frame basisby the receiver, the transmission output of the transmitter can be setto a value at which a desired communications area can be realized.

[0044] Preferably, the gain controller comprises a data setting deviceand a voltage-controlled amplifier and can freely change a communicationarea by means of variation of an amplification gain. The communicationsarea can be varied by means of changing the gain of thevoltage-controlled amplifier.

[0045] Preferably, the gain controller comprises a data setting deviceand a voltage-controlled amplifier and can freely change a communicationarea by means of variation of an amplification gain. A communicationsarea can be varied by means of varying the amount of attenuation of thevoltage-controlled attenuator.

[0046] Preferably, the antenna has a function of adjusting the angle atwhich the antenna is disposed, by means of a signal output from thereceiving rate detector, and can freely change a communications area bymeans of changing the angle. The angle at which the antenna is mountedis changed, to thereby enable changing of a communications area.

[0047] According to the sixth aspect of the invention, a receivercomprises: an antenna for receiving a radio wave transmitted from atransmitter; frequency converter for converting into an intermediatefrequency a high-frequency signal received by way of the antenna; ademodulation section for demodulating the intermediate frequency;decoder for converting the demodulated signal into digital data; andreceiving rate detector for detecting a receiving rate for each offrames of the received signal, wherein a communications area can befreely changed by means of changing a transmission output of thetransmitter on the basis of the receiving rate for each frame detectedby the receiving rate detector. On the basis of a receiving rateobtained on a per-frame basis, a transmission output of the transmittercan be set such that a desired receiving area is realized.

[0048] According to the seventh aspect of the invention, a road antennacomprises: a road antenna which is disposed at an elevated positionabove a road and establishes radio communication with an on-vehicledevice mounted in a vehicle traveling on the road; Doppler signalprocessor which detects the traveling direction of the vehicle on thebasis of a change arising in the frequency of a reflected wave due tothe Doppler effect, the reflected wave being formed when a transmissionwave emitted from the road antenna is reflected by the vehicle; andcontroller for inhibiting establishment of communication with a vehicletraveling in the lane opposite to the lane in which the detected vehicleis traveling. A transmission wave is transmitted from the road antennadisposed at an elevated position on the road, and the vehicle reflectsthe transmission wave, to thereby produce a reflected wave. Thethus-reflected wave is received by the road antenna. From the reflectedwave, Doppler signals which shift in proportion the speed of the vehicleare detected, and the traveling direction of the vehicle is detected byutilization of the Doppler effect. Thus, radio communication isestablished with only a vehicle traveling in a lane of interest, andestablishment of communication with a vehicle traveling in the oppositelane is inhibited.

[0049] Preferably, the road antenna comprises reflected wave extractionmeans which receives the reflected wave produced when the transmissionwave emitted from the road antenna for establishing radio communicationand collecting a toll is reflected by the vehicle as well as a receiptwave emitted from the on-vehicle device mounted in the vehicle, tothereby extract only the reflected wave. By utilization of a reflectedwave produced when a transmission wave emitted to the on vehicle devicefor establishing radio communication and collecting a toll is reflectedby the vehicle, the traveling direction of the traveling vehicle isdetected by the Doppler effect, thereby inhibiting establishment ofcommunication with the vehicle traveling in the opposite lane.

[0050] According to the eighth aspect of the invention, a travel-speedsupport system comprises: on-vehicle radio device to be mounted in atraveling vehicle; an antenna which establishes radio communication withthe vehicle and is to be mounted in a position above a road; anddetermination means which is provided in the antenna and determineswhether or not the travel speed of the vehicle is appropriate for aspeed limit imposed on a road, on the basis of the travel speed of thevehicle and a signal corresponding to a reflected wave, the reflectedwave being produced as a result of a radio emitted from the antennabeing reflected by the vehicle when the vehicle approaches or departsfrom the antenna. A warning to reduce travel speed can be sent to adriver of a vehicle which is traveling in excess of a speed limit, tothereby limit the speed of a vehicle on a road interconnecting aturnpike to an ordinary road. As a result, the present invention canurge a driver to practice safe driving on a road interconnecting aturnpike and an ordinary road.

[0051] Preferably, the antenna comprises: receiver for receiving areflected wave, the reflected wave being produced when a radiotransmitted to the on-vehicle unit is reflected by the vehicle; anddetector for detecting a signal received by the receiver and the speedof the vehicle. The travel speed of the vehicle can be limited on thebasis of the received signal and the detected travel speed of thevehicle. A

[0052] Preferably, the antenna comprises: speed warning means whichcompares the travel speed of the vehicle detected by the detector with apredetermined warning speed, determines whether or not the speed of thevehicle exceeds the warning speed, and issues a warning to the vehicleif the vehicle exceeds the warning speed. A warning message can be sentto the driver of a vehicle which is traveling in excess of a speedlimit, on the basis of the received signal and the detected travel speedof the vehicle.

[0053] The present invention provides an antenna for use with atravel-speed support system, comprises on-vehicle radio device to bemounted in a traveling vehicle; an antenna which establishes radiocommunication with the on-vehicle radio device and is to be disposed ata position above a road; and measurement means for measuring the speedof the traveling vehicle on the basis of a signal corresponding to areflected wave by means of the Doppler effect when the vehicleapproaches or departs from the antenna, the reflected wave beingproduced when a radio wave is reflected by the vehicle, wherein the roadincludes both a turnpike and an ordinary road. A limit is imposed on adriver of a vehicle which is traveling in excess of a speed limit, tothereby prevent a car accident. Thus, the present invention can enablethe driver to ascertain that his vehicle is traveling in excess of aspeed limit and send a warning to the driver. As a result, a cartraveling in excess of a speed limit imposed on a turnpike or anordinary road can be prevented.

[0054] Preferably, the antenna comprises: receiver for receiving a wavewhich is reflected by the vehicle, as a result of a radio wave beingtransmitted to the on-vehicle radio device; and detector for detectingthe signal received by the receiver and the speed of the vehicle. Alimit can be imposed on the speed of a vehicle on the basis of areceived signal and the detected speed of the vehicle.

[0055] Preferably, the antenna comprises: speed warning means whichcompares the travel speed of the vehicle as detected by the detecterwith a predetermined warning speed, determines whether or not the speedof the vehicle exceeds the warning speed, and issues a warning to thevehicle if the vehicle exceeds the warning speed. A warning can be sentto a driver of a vehicle which is traveling in excess of a speed limit,on the basis of a received signal and the detected speed of the vehicle,to thereby cause the driver to ascertain that his vehicle is travelingin excess of a speed limit.

[0056] According to the ninth aspect of the invention, a road antennacomprises: a road antenna which is disposed at an elevated position on aroad and sets a predetermined communications area on the road; and aroof-shaped structure which is located at an elevated position above theroad antenna, the side of the structure opposite the road antenna beingprovided with a radio-wave absorbing material, wherein radiocommunication is established between the road antenna and an on-vehicledevice mounted in a vehicle traveling on the road and within thecommunications area. Preferably, as the radio-absorbing member there maybe used a sheet-like radio-wave absorbing member, a paint-likeradio-wave absorbing member, or a multilayer radio-absorbing member.

[0057] A radio wave emitted from the road antenna is reflected by aroad, and the thus-reflected radio wave is absorbed by the radio-waveabsorbing member provided on the roof-shaped structure. As a result,there is formed a narrow communications area, which would also be formedwhen no structure is present above the road antenna.

BRIEF DESCRIPTION OF THE DRAWINGS

[0058]FIG. 1 is an illustration schematically showing the structure of aroad antenna according to first embodiment.

[0059]FIG. 2 is an external perspective view showing the road antenna.

[0060]FIG. 3 is a plan view showing a position onto which a laser beamis to be radiated when the position is located away from a shot target.

[0061]FIG. 4 is an illustration schematically showing the structure of aroad antenna according to a second embodiment.

[0062]FIG. 5 is a side elevation view schematically showing an electrictoll collection system to which a road antenna according to thirdembodiment.

[0063]FIG. 6 is a plan view showing the electric toll collection systemshown in FIG. 5.

[0064]FIG. 7 is a block diagram showing the road antenna according tothird embodiment.

[0065]FIG. 8 is a plan view showing an electric toll collection systemaccording to fourth embodiment of the present invention.

[0066]FIG. 9 is a block diagram showing the road antenna according tofourth embodiment.

[0067]FIG. 10 is a block diagram showing a radio system according tofifth embodiment.

[0068]FIG. 11 shows an frame format example employed in an electric tollcollection system.

[0069]FIGS. 12A and 12B are diagrams for describing an operation forsetting a communications area according to fifth embodiment.

[0070]FIG. 13 is a block diagram showing the configuration of atransmitter according to sixth embodiment.

[0071]FIG. 14 is a block diagram showing the configuration of atransmitter according to seventh embodiment of the present invention.

[0072]FIG. 15 is a diagram showing the configuration of a road antennaaccording to seventh embodiment.

[0073]FIG. 16 is a plan view showing the overall structure of a roadantenna according to eighth embodiment, wherein normal radiocommunication is established with a vehicle in a lane in which the roadantenna is disposed.

[0074]FIG. 17 is a plan view showing the overall structure of the roadantenna according to eighth embodiment, wherein establishment oferroneous communication with an oncoming vehicle in the opposite lane isprevented.

[0075]FIG. 18 is a block diagram showing the road antenna according toeighth embodiment.

[0076]FIG. 19 is a perspective general view showing the configuration ofa travel-speed support system according to ninth embodiment.

[0077]FIG. 20 is an illustration showing a relationship between aDoppler signal and the speed of a vehicle according to ninth embodiment.

[0078]FIG. 21 is a block diagram showing an antenna system according toninth embodiment.

[0079]FIG. 22 is an illustration showing a road antenna according totenth embodiment of the present invention.

[0080]FIG. 23 is a cross-sectional view for describing the principle onwhich a single layer radio-wave absorbing member absorbs a radio wave.

[0081]FIG. 24 is an illustration showing a road antenna according toeleventh embodiment.

[0082]FIG. 25 is an illustration showing a road antenna according to atwelfth embodiment of the present invention.

[0083]FIG. 26 is an enlarged view showing a multilayer radio-waveabsorbing member.

[0084]FIG. 27 shows an example of electric toll collection system.

[0085]FIG. 28 shows an example of communications area.

[0086]FIG. 29 is an illustration showing an example in which a roadantenna is mounted.

[0087]FIG. 30 shows an example distribution of level of receivingelectric field in a direction in which a vehicle is traveling.

[0088]FIG. 31 shows an example distribution of level of receivingelectric field in a direction in which a vehicle is traveling, when theangle at which the road antenna is mounted is changed.

[0089]FIG. 32 an explanatory view showing an example operation failureof the electric toll collection system.

[0090]FIG. 33 shows an example of communications area.

[0091]FIG. 34 is a block diagram showing the configuration of acommonly-used transmission circuit.

[0092]FIG. 35 is a diagram showing an example tollgate antenna employedin a turnpike ETC system.

[0093]FIG. 36 is a diagram showing an example directional pattern of aroad antenna.

[0094]FIGS. 37A and 37B show an example directional pattern of the roadantenna, wherein FIG. 37A is a graph showing a horizontal directionalpattern, and FIG. 37B is a graph showing a vertical directional pattern.

[0095]FIG. 38 is an illustration showing an example communications areaformed by a radio wave emitted from the road antenna.

[0096]FIG. 39 is an illustration showing reflection of a radio wave offa roof-shaped structure.

[0097]FIG. 40 is an illustration showing reflection of a radio wave offa mirror-image antenna.

[0098]FIG. 41 is an illustration showing an example communications areaformed by the radio wave reflected by the roof-shaped structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0099] Preferred embodiments will now be described with reference to thedrawings.

[0100] Embodiment 1

[0101]FIG. 1 is an illustration for schematically showing a road antennaaccording to the first embodiment of the present invention. In thedrawing, a road antenna 104 is disposed on a post 103 and at a positionelevated a predetermined height from a road surface. A laser-beamemitting device 111 is incorporated in the road antenna 104. The roadantenna 104 and the laser-beam emitting device 111 are connected to acontroller 112 disposed on a road R.

[0102]FIG. 2 is an external perspective view showing the road antenna104. The laser-beam emitting device 111 is incorporated in one corner ofa plane antenna surface 104 a. The direction in which the laser-beamemitting device 111 emits a laser beam is determined by an angle θ atwhich the road antenna 104 is mounted.

[0103] In the present embodiment, the direction in which the laser-beamemitting device 111 emits a laser beam (hereinafter referred to simplyas an “emission direction”) matches the orientation of the road antenna104. However, the emission direction of the laser-beam emitting device111 may differ from the orientation of the road antenna 104. Further, aplurality of laser-beam emitting devices 111 may be provided on the roadantenna 104.

[0104] The operation of the road antenna will now be described. At thetime of mounting of the road antenna 104, the road antenna 104 actuallyemits a radio wave, thus determining the distribution of electric fieldover the road R. On the basis of the determination result, the roadantenna 104 ascertains the communications area F101 and the non-responsearea F102. An angle θ at which the road antenna 104 is mounted andtransmission power are adjusted so as to comply with specifications.

[0105] When the communications area F101 and the non-response area F102are embodied, a laser beam is emitted from the laser-beam emittingdevice 111. A target mark 113 is provided at a predetermined location onthe road R onto which the laser beam is to be radiated.

[0106] In a case where no change arises in an angle θ at which the roadantenna 104 is mounted, a position at which the laser beam is radiated(hereinafter referred to as a “shot position 114”) remains unchanged andis situated on the target mark 113. In contrast, if a change arises inan angle θ at which the road antenna 104 is mounted, the shot position14 is moved away from the target mark 113. FIG. 3 is a plan view showingthe shot position 114 located away from the target mark 113.

[0107] Since the height “h” of the position where the road antenna 104is disposed is known, a deviation from the mount angle θ of the roadantenna 104 can be readily processed from the distance between thetarget mark 113 and the shot position 114. The communications area F101and the non-response area F102 can be estimated from the thus-processeddeviation from the mount angle θ. If the ETC system may have a chance ofyielding a failure, the mount angle θ of the road antenna 104 can becorrected.

[0108] Embodiment 2

[0109]FIG. 4 is an illustration schematically showing the configurationof a road antenna according to the second embodiment of the presentinvention. Those elements which are the same as those described inconnection with the first embodiment are assigned the same referencenumerals.

[0110] In the present embodiment, at the time of installation of theroad antenna 104, a laser-beam receiving device 115 is situated at thepredetermined shot position 113 on the road R for receiving the laserbeam emitted from the laser-beam emitting device 111. The laser-beamreceiving device 115 is connected to the controller 112. In otherrespects, the road antenna A according to the present embodiment isidentical in structure with that employed in the first embodiment.

[0111] In a case where a change arises in the mount angle θ of the roadantenna 104, the laser-beam receiving device 115 fails to receive thelaser beam emitted from the laser-beam emitting device 111. Informationabout such an operation failure is transmitted to the controller 112,and the controller 112 stops the operation of the road antenna 104. Ifthe operation of the road antenna 104 does not need to be stopped, thecontroller 112 may perform the function of sending an alarm message toan operator of the ETC system.

[0112] In the configuration of the road antenna shown in FIG. 4, it isexpected that, even if no change arises in the mount angle θ of the roadantenna 104, a laser beam is interrupted when the vehicle 101 istraveling over the road R, whereupon the laser-beam receiving device 115cannot receive a laser beam. For this reason, the influence ofinterruption of a laser beam on the ascertaining of receipt of a laserbeam, which would otherwise be induced by an obstacle, must beeliminated, on the basis of information about selection of position ofthe laser-beam receiving device 15 and information about the vehiclesensors 105.

[0113] Embodiment 3

[0114]FIG. 5 is a side elevation view schematically showing an electrictoll collection (ETC) system to which a road antenna according to thethird embodiment of the present invention is applied.

[0115] In the drawing, a road antenna 204 is disposed on a post 203 andat a position elevated a predetermined height from a road surface. Radiocommunication is established between the road antenna 204 and theon-vehicle device 202. Further, a radio controller 206 is disposed inthe vicinity of a post 203 and on one side of a road R. The radiocontroller 206 is connected to the road antenna 204 via a control line207.

[0116] A receiver 208 for receiving a radio wave emitted from the roadantenna 204 is disposed at a predetermined location on the surface ofthe road R. The receiver 208 is connected to the radio controller 206via a connection line 209.

[0117]FIG. 6 is a plan view of an electric toll collection (ETC) systemshown in FIG. 5. The communications area F21 is a range in which radiocommunication can be established between the on-vehicle device 202 andthe road antenna 204. The receiver 208 is disposed at a predeterminedposition on the road R and within the communications area F21.

[0118]FIG. 7 is a block diagram showing the configuration of the roadantenna 204 according to the present embodiment. The road antenna 204comprises an antenna section 241, a variable amplifier 242., and asignal source 243 of 5.8 GHz band.

[0119] Further, the radio controller 206 comprises an analog-to-digitalconversion section 261 for converting, into a digital signal, a signalentered by the receiver 208 by way of the connection line 209; aprocessing section 262; and a digital-to-analog conversion section 263for converting, into an analog signal, a signal output from theprocessing section 262. The receiver 208 comprises an antenna section281 for receiving a radio wave output from the road antenna 204; areceiving section 282; and a detection circuit 283 for detecting areceived radio wave.

[0120] The radio wave emitted from the antenna section 241 of the roadantenna 204 is received by an antenna section 281 and a receivingsection 282 of the receiver 208. In the receiver 208, a detectioncircuit 283 detects the received radio wave and outputs a voltage signalproportional to receiving power to the radio controller 206.

[0121] In the radio controller 206, the voltage signal output from thedetection circuit 283 by way of the control line 209 is converted into adigital signal by means of the analog-to-digital conversion section 261.The processing section 262 determines transmission power and outputscontrol data to be used for adjusting the transmission power of the roadantenna 204. The control data are delivered to the digital-to-analogconversion section 263, where the data are converted into an analogcontrol signal.

[0122] The thus-converted analog control signal is used for controllingthe degree of amplification of the variable amplifier 242. An initialvalue of transmission power is stored in the processing section 262beforehand. The degree of amplification of the variable amplifier 242 iscontrolled through use of a feedback loop until transmission powerbecomes close to the initial value, thereby maintaining constant thetransmission power of the road antenna 204 used for transmitting a radiowave.

[0123] Embodiment 4

[0124]FIG. 4 is a side elevation view schematically showing an electrictoll collection (ETC) system to which a road antenna according to thefourth embodiment of the present invention is applied. Those referencenumerals which are the same as those described in connection with thethird embodiment are assigned the same reference numerals.

[0125] In the present embodiment, four receivers 208A, 208B, 208C, and208D are disposed at corresponding four corners of the communicationsare F21 formed on the road R. In other respects, the ETC system isidentical in structure with that employed in the third embodiment.

[0126]FIG. 5 is a block diagram showing the structure of a road antennaaccording to the fourth embodiment. The receivers 208A through 208Ddisposed at the respective four corners of the communications are a F21are connected to the radio controller 206 by way of correspondingcontrol lines 209A through 209D.

[0127] Each of the receivers 208A through 208D comprises an antennasection 281, a receiving section 282, and a detection circuit 283. Theradio controller 206 has the analog-to-digital conversion section 261for converting into a digital signal a voltage signal output from thedetection circuit 283 of each of the receivers 208A through 208D. Theanalog-to-digital conversion section 261 is formed from, for example,four analog-to-digital converters which are arranged in a side-by-sideconfiguration.

[0128] The radio wave emitted from the antenna section 241 of the roadantenna 204 is received by the antenna section 281 and the receivingsection 282 of each of the receivers 208A through 208D. The detectioncircuit 283 detects the radio wave received by each of the receivers208A through 208D and outputs a voltage signal proportional to thereceiving power used for receiving the radio wave is output to the radiocontroller 206.

[0129] The radio controller 206 receives the voltage signal which isoutput from the detection circuits 283 of each of the receivers 208Athrough 208D by way of a corresponding one of the connection lines 209Athrough 209D. The thus-received voltage signal is converted into adigital signal by the analog-to-digital conversion section 261. Theprocessing section 262 compares a predetermined value with four digitalsignals, and the angle at which the road antenna 204 is mounted isdetected on the basis of a comparison result.

[0130] For example, in a case where the voltage signals output from thereceivers 208A and 208D are large-and the voltage signals output fromthe receivers 208B and 208C are small, it is determined that the roadantenna 204 is inclined to left with respect to the direction in whichthe vehicle 201 is traveling. If a great inclination has arisen in theroad antenna 204, a radio wave may interfere with radio communicationestablished by a vehicle which is traveling on an adjacent lane. Inorder to prevent such an interference, an alarm is issued.

[0131] Embodiment 5

[0132]FIG. 10 is a diagram showing the structure of a radio systemaccording to the fifth embodiment of the present invention, the systemadopting an ASK (amplitude shift keying) scheme.

[0133] In FIG. 10, reference numeral 301 designates a transmissionsection; 311 designates an ASK (amplitude shift keying) modulationsection; 312 designates gain control section; 313 designates a poweramplification section; and 314 designates an antenna. The gain controlsection 312 is made up of a voltage-controlled amplifier 312 a and adata setting device 312 b.

[0134] Reference numeral 302 designates a receiving section of otherparty; 321 designates an antenna; 322 designates frequency conversionsection; 323 designates an ASK (amplitude shift keying)demodulation-section; and 324 designates decode section. The decodesection 324 is made up of a demodulator 324 a and receiving ratedetermination means 324 b.

[0135] The operation of a transmission output control circuit having theforegoing configuration will now be described. In the transmissionsection 301, an ASK (amplitude shift keying) modulation signal producedby the ASK modulation section 311 is amplified to a desired level by thepower amplification section 313 after having passed through the gaincontrol section 312. The thus-amplified signal is transmitted as a radiowave from the antenna 314. The gain control section 312 determines thegain of the voltage-controlled amplifier 312 a in accordance with thesettings of the data setting device 312 b.

[0136] The receiving section 302 is disposed at an arbitrary location inthe lane 363 shown in FIG. 35 and performs a receiving operation. InFIG. 10, a high-frequency signal received by the antenna 321 isconverted into an intermediate frequency by means of the frequencyconversion means 322, and the intermediate frequency is demodulated intoan ASK (amplitude shift keying) signal by the ASK demodulation section323. The thus-demodulated signal is converted into digital data by thedemodulator 324 a of the decode section 324. Simultaneously, thereceiving rate determination means 324 b determines, on a per-framebasis, whether or not the received signal is correct transmission data.

[0137]FIG. 11 shows an example frame format employed in the ETC system.The receiving section 302 shown in FIG. 10 receives an FCMS slot andeither an MDS(1) slot or an MDS(3) slot shown in FIG. 11. Each slotcontains an error detection code of 16-bit CRC (cyclic redundancy check)and determines whether or not received data are correct data.

[0138] By reference to FIGS. 12A and 12B, the control of a transmissionoutput of the transmission section 1 shown in FIG. 10 will now bedescribed. In the antenna shown in FIGS. 12A and 12B, reference numeral331 designates an area covered by a road antenna 361; 332 designates adesired communications area; and 302 designates the receiving section302.

[0139]FIG. 12A shows a situation in which the receiving section 302located within the desired communication area 332 cannot establishcommunication, because the coverage area 331 formed by the road antenna361 is narrow. At this time, the result of the measurement performed bythe receiving rate determination means 324 b of the receiving section302 shows that communication is not feasible. In order to enablecommunication, the data setting device 312 b of the transmission section301 shown in FIG. 10 is reset. The gain of the voltage-controlledamplifier 312 a is increased until the result of the measurementperformed by the receiving rate determination means 324 b of thereceiving section 302 shows that communication is feasible. Thereceiving rate determination means 324 b measures a receiving rate on aper-frame basis, and the gain (transmission output) of thevoltage-controlled amplifier 312 a is fixedly set while the measurementresult shows that communication is feasible. As a result, the coveragearea 331 formed by the road antenna 361 is correctly set while thereceiving section. 302 is located within the desired communications area332, thereby rendering the entirety of the desired communications area332 receivable.

[0140]FIG. 12B shows a situation in which the receiving section 302located outside the communications area 332 has establishedcommunication because of the wide coverage area formed by the roadantenna 361. At this time, the result of the measurement performed bythe receiving rate determination means 324 b of the receiving section302 shows that communication is feasible. In this case, the data settingdevice 312 b of the transmission section 301 is reset, and the receivingrate determination means 324 b of the receiving section 302 measures areceiving rate on a per-frame basis. The gain of the voltage-controlledamplifier 312 a is decreased until the measurement result shows thatcommunication is not feasible. The gain of the voltage-controlledamplifier 312 a is fixedly set while the result of the measurementperformed by the receiving rate determination means 324 b shows thatcommunication is not feasible. As a result, the coverage area 331 formedby the road antenna 361 is appropriately set so that the receivingsection 302 located outside the desired communications area 332 becomesunreceivable.

[0141] In this embodiment, the receiving rate determination means 324 bof the receiving section 302 measures a receiving rate on a per-framebasis, and the gain of the voltage-controlled amplifier 312 a of thetransmission section 301 is controlled on the basis of the measurementresult, thereby ensuring the desired communications area 332.

[0142] Embodiment 6

[0143] The sixth embodiment of the present invention will now bedescribed by reference to a block diagram shown in FIG. 13. Asillustrated, the transmission section of the present embodiment isidentical in configuration with that shown in FIG. 10, except that theconfiguration of the gain control section 312 is changed. Explanation ofthe identical configuration is omitted here. The gain control section312 according to the sixth embodiment is made up of an amplifier 312 cand a voltage-controlled attenuator 312 d.

[0144] In such a configuration, the amount of attenuation of thevoltage-controlled attenuator 312 d of the transmission section 301 isdetermined in accordance with the settings of the data setting device312 b, thereby setting a transmission output. The receiving ratemeasurement means 324 b of the receiving section 302 measures areceiving rate on a per-frame basis. The amount of attenuation of thevoltage-controlled attenuator 312 d of the transmission section 301 isvariably controlled, thereby ensuring the desired communications area332. At this time, it is recommendable to ensure the desired receivingarea 332 in accordance with procedures analogous to those employed inthe setting example (FIG. 12) mentioned previously.

[0145] Embodiment 7

[0146] The seventh embodiment of the present invention will now bedescribed. FIG. 14 is a block diagram showing another exampleconfiguration of the transmission section 301. According to the seventhembodiment, as shown in FIG. 14, the transmission section 301 isadditionally provided with mount angle adjustment means 341. FIG. 15shows an example configuration of the road antenna 361. The road antenna361 comprises a gantry 366, a post 367, a mount angle adjuster 368, anda road antenna main unit 369.

[0147] In the above-described configuration, the angle of the mountangle adjuster 368 is determined in accordance with the settings of themount angle adjustment means 341. The receiving rate measurement means324 b of the receiving section 302 receives a receiving rate on aper-frame basis, thereby ensuring a desired communications area. Morespecifically, the road antenna 361 has a directional pattern such asthat shown in FIG. 36. A communications area is moved by means ofchanging the mount angle of the road antenna 361. On the basis of thereceiving rates which have been 42. measured on a per-frame basis, themount angle adjuster 368 adjusts the angle of the road antenna main unit369, by means of varying the settings of the mount angle adjustmentmeans 341 such that the desired communications area 332 is achieved.

[0148] The present invention is not limited to the above-describedembodiments, and the ASK modulation section, the gain controller, thepower amplification section, the antenna, the frequency conversionmeans, the ASK demodulation section, the decode means, and the mountangle adjustment means can be modified variously within the scope of theinvention.

[0149] Although the previous embodiments have described a radio systemadopting an amplitude shift keying (ASK) scheme, the present inventioncan also be applied to a frequency shift keying (FSK) scheme or a phaseshift keying (PSK) scheme. For example, if an FSK modulation section forgenerating an FSK modulation signal is employed as a substitute for theASK modulation section 311 and an FSK demodulation section fordemodulating an FSK modulation signal is employed as a substitute forthe ASK demodulation section 323, a radio system of FSK scheme can beemployed. Similarly, when a PSK modulation section is employed as asubstitute for the ASK modulation section 311 and a PSK demodulationsection is employed as a substitute for the ASK demodulation section323, a radio system of PSK scheme can be employed.

[0150] Embodiment 8

[0151]FIGS. 16 and 17 are plan view showing the overall structure of aroad antenna according to the eighth embodiment of the presentinvention. FIG. 16 shows normal radio communication established with avehicle traveling in a lane in which the antenna is disposed, and FIG.17 shows prevention of erroneous communication with a vehicle travelingon the opposite lane.

[0152] As shown in FIGS. 16 and 17, a road antenna 404 mounted on a post403 transmits a transmission wave Wt to a vehicle 401 and receives areceipt wave transmitted from an on-vehicle device 402 mounted in thevehicle 401, thereby establishing radio communication with theon-vehicle device 402. Simultaneously, the transmission wave Wt isreflected by the vehicle 401, thereby causing a reflected wave Wf. Theroad antenna 404 also receives the reflected wave Wf.

[0153] As an undulation source (i.e., the traveling vehicle 401)approaches an observer (i.e., the road antenna 404), the frequency ofthe reflected wave Wf becomes greater than that of the transmission waveWt. In contrast, as the undulation source departs from the observer, thefrequency of the reflected wave Wf becomes lower than that of thetransmission wave Wt. The traveling direction of the traveling vehicle401 can be processed through such use of the Doppler effect.Consequently, if the vehicle 401 is traveling on the opposite lane, theantenna system 404 can prevent establishment of radio communication withthe on-vehicle device 402 mounted in the vehicle 401.

[0154]FIG. 18 is a block diagram showing the configuration of the roadantenna according to the eighth embodiment. In this drawing, thetransmission wave Wt output from the transmission section 411 is outputto only the antenna section 413 by means of a circulator 412. Theantenna section 413 transmits the transmission wave Wt to the outside ofthe road antenna 404.

[0155] After the transmission wave Wt has been received by theon-vehicle device 402 mounted in the vehicle 401, the antenna section413 receives the receipt wave Wr transmitted from the on-vehicle device402 and the reflected wave Wf (Wt±Δ) which results from the transmissionwave Wt being reflected by the vehicle 401 and shifts in proportion tothe speed of the vehicle 401. The thus-received waves are output to afilter section 414 by the circulator 412.

[0156] The filter section 414 permits passage of only the reflected waveWf after having removed the receipt wave Wr. The reflected wave Wf ismixed with the transmission wave Wt by means of an orthogonaldemodulator 415, to thereby extract Doppler signals; that is, signal Iand signal Q which shift in proportion to the speed of the vehicle 401.The Doppler signals are sent to a Doppler signal processing section 416.

[0157] The Doppler signal processing section 416 detects the travelingdirection of the vehicle 401 which causes the reflected wave Wf. Sinethe Doppler signals; that is, signals I and Q, advance or lag dependingon the traveling direction of the vehicle 401. Therefore, the travelingdirection of the vehicle 401 can be detected on the basis of the phaserelationship between I and Q signals.

[0158] The thus-detected traveling direction is output to a controlsection 417. The control section 417 inhibits establishment of radiocommunication with a vehicle in the opposite lane, the vehicle travelingaway from the road antenna 404 (i.e., a signal relating to the travelingdirection of the vehicle shows that the vehicle moves away).

[0159] Embodiment 9

[0160] A ninth embodiment of the present invention will now be describedby reference to FIGS. 19 and 20. FIG. 19 is an outline showing thestructure of the present invention. As shown in FIG. 19, an antenna 504is mounted at a center plate 503A of a post 503, and on-vehicle radiodevice 502 is mounted in a traveling vehicle 501.

[0161] The antenna 504 transmits a transmission wave Wt to theon-vehicle radio device 502 of the traveling vehicle. 501 and receives areceipt wave Wr transmitted from the on-vehicle radio device 502, thusestablishing radio communication with the on-vehicle radio device 502.Simultaneously, the antenna 504 receives a reflected wave Wf whicharises when the transmission wave Wt is reflected by the travelingvehicle 501.

[0162] In the present embodiment, as an undulation source (i.e., thetraveling vehicle 501) approaches an observer (i.e., the antenna 504),the frequency of the reflected wave Wf becomes greater than that of thetransmission wave Wt. In contrast, as the undulation source departs fromthe observer, the frequency of the reflected wave Wf becomes lower thanthat of the transmission wave Wt. The travel speed of the travelingvehicle 501 can be processed through such use of the Doppler effect.

[0163] Information about the travel speed of the traveling vehicle 501is transmitted to a speed warning machine 506 installed on a road, or tothe on-vehicle radio device 502 mounted on the traveling vehicle 501, tothereby send a warning to only a vehicle which is traveling at highspeed.

[0164]FIG. 20 shows the principle on which the speed of a travelingvehicle is measured through use of the Doppler effect. An antenna 504mounted on a post 503 receives a reflected wave Wf which is producedwhen a transmission wave Wt output from the antenna 504 is reflected bythe traveling vehicle 501.

[0165] For instance, provided that an angle 0 at which the transmissionWt enters the traveling vehicle 501 is taken, a travel speed V of thetraveling vehicle 21 is usually expressed by the following equation.

V=2c·fd/ft·cos θ

[0166] where c represents the speed of light, ft represents atransmission frequency, and fd is a Doppler frequency.

[0167] Provided that θ=0 (deg.) and “ft” is 5.8 GHz, the travel speed ofthe vehicle is processed on the basis of the fact that a travel speed of1 km/h equivalents to a Doppler frequency of 10.75 Hz.

[0168]FIG. 21 is a block diagram showing an antenna according to thisembodiment. The transmission wave Wt output from a transmission section537 is delivered to solely an antenna section 510 by means of acirculator 511 shown in FIG. 21.

[0169] A transmission wave Wt is delivered to the antenna section 510.After the transmission wave Wt has been received by the on-vehicle radiodevice 502 mounted in the traveling vehicle 501, the transmission wavewt output from the on-vehicle radio device 502 and a reflected waveWf-which is reflected by the traveling vehicle 501 and is shifted inproportion to the travel speed of the traveling vehicle-are received bythe antenna section 510 and delivered to a filter section 512 by meansof the circulator 511.

[0170] The filter section 512 eliminates a received wave Wr and permitspassage of only a reflected wave Wf. A mixer 513 mixes the reflectedwave Wf with the transmission wave Wt, to thereby extract only a Dopplersignal 550 which is shifted in proportion to the travel speed of thevehicle. The Doppler signal 550 is delivered to a Doppler signalprocessing section 514. Determination means 520 is essentially made upof the Doppler signal processing section 514, a control section 515, anda comparator 516.

[0171] The Doppler signal processing section 514 processes the travelspeed of the vehicle which has produced the reflected wave Wf. Since theDoppler signal 550 is shifted in proportion to the speed of the vehicle501, the speed of the vehicle 501 can be determined by means ofmeasuring the frequency of the Doppler signal 550. The thus-determinedspeed is output as speed information 560 to the control section 515.

[0172] The speed information 560 and a warning speed 570 previously setto a storage section 518 are input to the comparator 516, where thespeed information 0.560 is compared with the warning speed 570. Theresult of comparison is output to the control section 515. The controlsection 515 issues a warning message to the vehicle 501 from the speedwarning machine 506 in a case where the result output from thecomparator 516 is positive.

[0173] More specifically; a warning signal 580 is sent to thetransmission section 537 of the on-vehicle radio device 502 mounted inthe vehicle 501, wherewith the on-vehicle radio device 502 issues awarning message, to thereby-urge a driver to reduce the travel speed.

[0174] Embodiment 10

[0175]FIG. 22 is an illustration showing a road antenna according to thetenth embodiment of the present invention. As shown in FIG. 22, avehicle 601 is equipped with an on-vehicle device 602, and a roadantenna 604 is mounted on a post 603 and at an elevated position above aroad R. Radio communication is established between the on-vehicle device602 and the road antenna 604. A sheet-like thin radio-wave absorbingmember 612 is laid on the underside of a roof 611 disposed at anelevated position above the road antenna 604.

[0176]FIG. 23 is a cross-sectional view for describing the principle onwhich a single layer radio-wave absorbing member constituting the thinradio-wave absorbing member 612 absorbs a radio wave.

[0177] As shown in FIG. 23, the thin radio-absorbing member 612 isformed by stacking a metal plate 612 a on absorbing material 612 b. Whena radio wave of field Eo enters the absorbing material 612 b, field Er1is reflected by the absorbing material 612 b, and a remaining portion ofthe radio wave passes through the inside of the absorbing material 612b. The absorbing material 612 b may be formed of resistive fiber, FRP,rubber ferrite, or rubber carbon.

[0178] The radio wave which has entered the inside of the absorbingmaterial 612 b is attenuated in the form of an exponential function, byvirtue of the attenuation factor of the absorbing material 612 b.However, the radio wave is not sufficiently reduced, and hence the radiowave is totally reflected by the metal plate 612 a. The radio wave thathas been totally reflected reaches the surface of the absorbing material612 b while being attenuated by the absorbing material 612 b. A portionof the thus-attenuated radio wave is reflected by a boundary surfacebetween the surface of the absorbing material 612 b and the insidethereof, and the thus-reflected portion enters the inside of theabsorbing material 612 b. The remaining portion of the radio wave goesout the absorbing material 612 b, thus generating field Er2 whichcorresponds to the radio wave reflected by the absorbing material 612 b.

[0179] The radio wave is repeatedly subjected to the foregoing steps,thereby causing reflected radio waves to propagate toward the road.Every time the radio wave travels through the inside of the absorbingmaterial 612 b, the intensity of electric field of the radio wave isgradually reduced as the radio wave is reflected by the thinradio-absorbing member 612.

[0180] If the first reflected field Er1 and the second reflected fieldEr2 are caused to become equal in intensity and opposite in phase, thereflection factor of the absorbing material 612 b becomes zero. However,a single reflection of a radio wave off the metal plate 612 a isinsufficient in practice, and consideration must be given to multiplereflections of a radio wave of the metal plate 612 a. As mentionedabove, the radio-wave absorbing material 612 b has the function ofattenuating an electric field and delaying the phase of the electricfield.

[0181] The operation of the road antenna according to the tenthembodiment will now be described. The road antenna 604 is disposed at acertain elevated position above the road R and at a certain angle. Theroad antenna 604 is formed by means of a beam-shaping operation, has adirectional pattern, and radiates a-radio wave at a specifiedtransmission E.I.R.P level.

[0182] The radio wave emitted from the road antenna 6.04 forms thecommunications area F1 and is reflected by the road R. The radio wavereflected by the road R reaches the roof 611. The thin radio-waveabsorbing member 612 laid on the roof 611 absorbs the reflected radiowave, thus preventing reflection of the radio wave, which wouldotherwise be caused by the roof 611.

[0183] According to this embodiment, the thin radio-wave absorbingmaterial 612 is laid on a structure disposed at an elevated positionabove the road antenna 604. As a result, there is formed a narrowcommunications area, which would also be formed when no structure ispresent above the road antenna 604.

[0184] Embodiment 11

[0185]FIG. 24 is an illustration showing a road antenna according to theeleventh embodiment of the present invention. As illustrated, thevehicle 601 is equipped with the on-vehicle device 602, and the roadantenna 604 is mounted on the post 603 and at an elevated position abovethe road R. Radio communication is established between the on-vehicledevice 602 and the road antenna 604. A paint-type radio-wave absorbingmember 613 is laid on the underside of the roof 611 disposed at anelevated position above the road antenna 604. The paint-type radio-waveabsorbing member 0.613 is identical in absorption principle and materialwith the thin radio-absorbing member 612.

[0186] The road antenna 604 is disposed at a certain elevated positionabove the road R and at a certain angle. The road antenna 604 is formedby means of a beam-shaping operation, has a directional pattern, andradiates a radio wave at a specified transmission E.I.R.P level.

[0187] The radio wave emitted from the road antenna 604 forms thecommunications area F1 and is reflected by the road R. The radio wavereflected by the road R reaches the roof 611. The thin radio-waveabsorbing member 613 laid on the roof 611 absorbs the reflected radiowave, thus preventing reflection of a radio wave, which would otherwisebe caused by the roof 611.

[0188] According to the present embodiment, the paint-type radio-waveabsorbing material 613 is laid on a structure disposed at an elevatedposition above the road antenna 604. As a result, there is formed anarrow communications area, which would also be formed when no structureis present above the road antenna 604.

Embodiment 12

[0189]FIG. 25 is an illustration showing a road antenna according to thetwelveth embodiment of the present invention. As illustrated, thevehicle 1 is equipped with the on-vehicle device 602, and the roadantenna 604 is mounted on the post 603 and at an elevated position abovethe road R. Radio communication is established between the on-vehicledevice 602 and the road antenna 604. A wedged multilayer radio-waveabsorbing member 614 is laid on the underside of the roof 611 disposedat an elevated position above the road antenna 604.

[0190]FIG. 26 is an enlarged cross-section of the wedged multilayerradio-wave absorbing member 614. The wedged multilayer radio-waveabsorbing member 614 is formed by stacking, in the sequence given, awedge 14 a formed of an absorbing material, an intermediate multilayerabsorbing material 614 b, and a metal plate 614 c.

[0191] In terms of a frequency band or entrance characteristic, a singlelayer radio-wave absorbing member encounters a limitation. For thisreason, there is employed a multilayer structure, in which a materialhaving a material constant close to that of air is provided at aposition close to the surface of an absorbing member, and a materialhaving a greater radio-wave absorbing characteristic is provided in adeeper position of the absorbing member. Accordingly, there is achieveda broad radio-wave absorbing characteristic, in which, even if thefrequency of a reflected radio wave is changed slightly, the radio waveenters the inside of the absorbing member and is gradually attenuated.Further, the absorbing member is formed into a wedge or pyramidgeometry, thereby decreasing the surface area of the absorbing member.Even when an absorbing member is formed from-a single material, thedielectric constant of the absorbing member is equivalently reduced,thus achieving a dielectric constant close to that of air.

[0192] The operation of the road antenna according to the thirdembodiment will now be described. The road antenna 604 is formed bymeans of a beam-shaping operation, has a directional pattern, and isdisposed at a certain elevated position above the road R and at acertain angle. The road antenna 604 radiates a radio wave at a specifiedtransmission E.I.R.P level.

[0193] The radio wave emitted from the road antenna 604 forms thecommunications area Fl and is reflected by the road R. The radio wavereflected by the road R reaches the roof 611. The wedged multilayerradio-wave absorbing member 614 laid on the roof 611 absorbs thereflected radio wave, thus preventing reflection of a radio wave, whichwould otherwise be caused by the roof 611.

[0194] According to this embodiment, the wedged multilayer radio-waveabsorbing material 614 is laid on a structure disposed at an elevatedposition above the road antenna 604. As a result, there is formed anarrow communications area, which would also be formed when no structureis present above the road antenna 604.

[0195] As has been described, according to the present invention, anoffset in mount angle of a road antenna can be readily ascertained on aroad, on the basis of a target position onto which a laser beam is to beradiated and a position on which a laser beam is actually radiated. Solong as an angle of the road antenna is adjusted once per day, the roadantenna yields an advantage of maintaining the ability to correctlycollect a toll.

[0196] According to the present invention, a receiver detects thetransmission power of a radio wave output from a road antenna, and theroad antenna is subjected to feedback control on the basis of thethus-detected signal, thereby maintaining constant the power of a radiowave output from the road antenna. Consequently, the present inventionsuppresses occurrence of a change in a communications area, therebypreventing interference of radio communication established by a vehicletraveling in an adjacent lane and occurrence of a system failure.

[0197] Further, the communications area setting method according to thepresent invention enables setting of a desired communications area onthe basis of receiving rates, the receiving rates having been detectedby the receiving rate determination means of the receiver when atransmission output of the transmitter is changed setting of acommunications area does not involve a necessity for measuring a fieldintensity and can be performed readily.

[0198] The radio system according to the present invention is configuredso as to modulate/demodulate a transmission signal, to thereby detect areceiving rate which has been determined on a per-frame basis when thereceiving section demodulates digital data. As a result, a desiredcommunications area can be set by means of changing only a transmissionoutput of the transmission section on the basis of the receiving rate ofeach frame.

[0199] The transmitter according to the present invention modulates atransmission signal and can vary a communications area by means of atransmission output being variably controlled by the gain controller.Thus, a desired communications area can be set by means of varying atransmission output.

[0200] A communications area can be changed by means of varying aamplification gain of the voltage-controlled amplifier, the amount ofattenuation of the voltage-controlled attenuator, the angle at which theantenna is mounted, or a combination thereof.

[0201] The receiver according to the present invention detects areceiving rate on a per-frame basis at the time of demodulation of amodulated transmission signal and changes a transmission output of thetransmitter, thereby setting a desired communications area on the basisof a change in receiving rate.

[0202] Moreover, a transmission wave transmitted from a road antenna isreflected by a traveling vehicle, thus causing a reflected wave. Fromthe reflected wave, Doppler signals which shift in proportion to therelative speed of the traveling vehicle are detected. On the basis ofthe Doppler signals, the traveling direction of the traveling vehicle isdetected, thereby avoiding establishment of erroneous communication withan oncoming vehicle traveling in the opposite lane.

[0203] Further, a transmission wave sent from an antenna section isreflected by a vehicle, to thereby produce a reflected wave. A Dopplersignal which is shifted in proportion to a relative speed of the vehicleis detected by receipt of the reflected wave and determine the travelspeed of a traveling vehicle. Thus, the present invention can reduce thespeed of a traveling vehicle.

[0204] A warning to reduce a travel speed can be sent to a driver of avehicle which is traveling in excess of a speed limit. Accordingly, thepresent invention can assist in realization of safe travel on a roadinterconnecting a turnpike and an ordinary road.

[0205] Furthermore, according to the present invention, even when aroof-like structure is located at an elevated position above a roadantenna, a radio-absorbing member is provided on the structure, tothereby realize a narrow communications area, which would also be formedwhen no such structure is present.

What is claimed is:
 1. A road antenna apparatus comprising: a roadantenna disposed above a road and establishing radio communication withan on-vehicle radio device mounted in a vehicle; a receiver disposedwithin a communication area on the road; a controller which controls anoperation of the antenna based on a detection of the receiver.
 2. Theroad antenna apparatus as described in claim 1, further comprising: alaser-beam emitting device which is mounted on the road antenna andradiates a laser beam on the road, wherein the receiver is a laser-beamreceiving device which receives the laser beam emitted from thelaser-beam emitting device.
 3. The road antenna apparatus as describedin claim 1, wherein the controller controls a transmission output signallevel of the road antenna based on a receiving rate of the transmissionoutput signal level.
 4. A road antenna apparatus comprising: a roadantenna disposed above a road and establishing radio communication withan on-vehicle radio device mounted in a vehicle; and a laser-beamemitting device which is mounted on the road antenna and radiates alaser beam on the surface of the road.
 5. The road antenna apparatus asdescribed in claim 4, further comprising: a laser-beam receiving devicemounted on the road and receiving the laser beam emitted from thelaser-beam emitting device, wherein the operation of the road antenna isstopped when the laser-beam receiving device cannot receive the laserbeam.
 6. A road antenna apparatus comprising: a road antenna above aroad and establishing radio communication with an on-vehicle radiodevice mounted in a vehicle; a receiver disposed within a communicationarea on the road, and receiving a radio wave output from the roadantenna, and outputs a signal proportional to the power of the radiowave; and a controller for determining transmission power of the roadantenna on the basis of the signal output from the receiver, thecontroller controls the road antenna so as to prevent the transmissionpower of the road antenna from exceeding a predetermined value.
 7. Theroad antenna as described in claim 6, wherein receivers are disposed atrespective corners of the communications area, wherein the controllerdetermines the angle at which the road antenna is mounted based onsignals output from the respective receivers, the controller detects anoffset in the angle of the antenna with respect to a predeterminedangle.
 8. A method of setting a communications area, comprising thesteps of: measuring a receiving rate for each of frames of a receivedsignal when a receiver receives a radio wave transmitted from atransmitter; detecting change in receiving rate on a per-frame basis,the change being induced by a change in a transmission output of theradio wave transmitted from the transmitter; and setting, into thetransmitter, a transmission output obtained when there is detected areceiving rate suitable for a desired communications area establishedbetween the transmitter and the receiver.
 9. A radio system comprising:a transmission section including a modulation section for producing amodulation signal; a gain controller which controls a transmissionoutput, a power amplification section for amplifying a transmissionsignal to a desired level, and an antenna; and a receiving sectionincluding: an antenna; a frequency converter which converts into anintermediate frequency a high-frequency signal received by way of theantenna; a demodulation section which demodulates the intermediatefrequency, a decode section which converts a demodulated signal intodigital data; and a receiving rate detector which detects a receivingrate for each of frames of a received signal.
 10. A transmittercomprising: a modulation section which produces a modulation signal; again controller which controls a transmission output; a poweramplification section which amplifies a transmission signal to a desiredlevel; and an antenna, wherein the gain controller varies thetransmission output on the basis of a receiving rate for each framedetermined when a receiver receives a transmission signal.
 11. Thetransmitter as described in claim 10, wherein the gain controllerincludes a data setting device and a voltage-controlled amplifier, andcan freely change a communication area by means of variation of anamplification gain.
 12. The transmitter as described in claim 10,wherein the gain controller includes a data setting device and avoltage-controlled attenuator, and can freely change a communicationarea by means of variation of an attenuation.
 13. The transmitter asdescribed in claim 10, wherein the antenna has a function of adjustingthe angle at which the antenna is disposed, by means of a the receivingrate and can freely change a communications area by means of changingthe angle.
 14. A receiver comprising: an antenna for receiving a radiowave transmitted from a transmitter; a frequency converter whichconverts into an intermediate frequency a high-frequency signal receivedby way of the antenna; a demodulation section which demodulates theintermediate frequency; decode section which converts the demodulatedsignal into digital data; and receiving rate detector which detects areceiving rate for each of frames of the received signal, wherein acommunications area can be freely changed by means of changing atransmission output of the transmitter on the basis of the receivingrate for each frame detected by the receiving rate detector.
 15. A roadantenna apparatus comprising: a road antenna disposed above a road andestablishing radio communication with an on-vehicle device mounted in avehicle; Doppler signal processor which detects the traveling directionof the vehicle on the basis of a change arising in the frequency of areflected wave due to the Doppler effect, the reflected wave beingformed when a transmission wave emitted from the road antenna isreflected by the vehicle; and control means which allows establishmentof communication with the vehicle traveling in a predetermineddirection.
 16. The road antenna apparatus as described in claim 15,further comprising: reflected wave extraction means which receives thereflected wave and a receipt wave emitted from the on-vehicle device,the reflected wave extraction means extracts only the reflected-wave.17. A travel-speed support system comprising: an on-vehicle radio devicemounted in a vehicle; an antenna mounted above a road and establishingradio communication with the on-vehicle radio device; and determiningmeans provided in the antenna and determining whether the travel speedof the vehicle is appropriate for a speed limit imposed on a road, onthe basis of the travel speed of the vehicle based on a signalcorresponding to a reflected wave, the reflected wave being produced asa result of a radio emitted from the antenna being reflected thevehicle.
 18. The travel-speed support system as described in claim 17,wherein the antenna includes: a receiver which receives a reflectedwave, the reflected wave being produced when a radio transmitted to theon-vehicle radio device is reflected from the vehicle; and a detectorwhich detects a signal received by the receiver and the speed of thevehicle.
 19. The travel-speed support system as described in claim 17,wherein the antenna includes: speed warning means which compares thetravel speed of the vehicle detected by the detector with apredetermined warning speed, determines whether the speed of the vehicleexceeds the warning speed, and issues a warning to the vehicle if thevehicle exceeds the warning speed.
 20. An antenna for use with atravel-speed support system, comprising: on-vehicle radio device mountedin a traveling vehicle; an antenna disposed at a position above a road,and establishing radio communication with the on-vehicle radio device;and a speed detector which measures the speed of the traveling vehicleon the basis of a signal corresponding to a reflected wave by means ofthe Doppler effect when the vehicle approaches or departs from theantenna, the reflected wave being produced when a radio wave isreflected by the vehicle, wherein the road includes both a turnpike andan ordinary road.
 21. The antenna as described in claim 20, wherein theantenna includes: a receiver which receives a wave which is reflected bythe vehicle, as a result of a radio wave being transmitted to theon-vehicle radio device; and a detector which detects the signalreceived by the receiving means and the speed of the vehicle.
 22. Theantenna as described in claim 20, wherein the antenna includes: a speeddetector which compares the travel speed of the vehicle as detected bythe detection means with a predetermined warning speed, determineswhether or not the speed of the vehicle exceeds the warning speed, andissues a warning to the vehicle if the vehicle exceeds the warningspeed.
 23. A road antenna comprising: a road antenna disposed above aroad and setting a communications area on the road; and a structurewhich is located above the road antenna, the side of the structureopposite the road antenna being provided with a radio-wave absorbingmaterial, wherein radio communication is established between the roadantenna and an on-vehicle device mounted in a vehicle traveling on theroad and within the communications area.
 24. The road antenna asdescribed in claim 23, wherein the radio wave absorbing member is aradio-wave absorbing member.
 25. The road antenna as described in claim23, wherein the radio wave absorbing member is a paint-like radio-waveabsorbing member.
 26. The road antenna as described in claim 1, whereinthe radio-absorbing member is a multilayer radio-absorbing member.